U.S. Pat. No. 1,413,943 issued Apr. 25, 1922 to J. C. Snyder, U.S. Pat. No. 1,514,872 issued Nov. 11, 1924 to C. E. Starr, and U.S. Pat. No. 2,331,684 issued Oct. 12, 1943 to J. E. Henningsen are generally illustrative of planetary gear mechanisms that utilize a manually shifted internal ring gear element. In a first axial position the ring gear is nonrotatably connected to the housing of the mechanism to provide a speed reduction ratio at the output, while in a second axial position thereof the ring gear is connected for joint rotation with the planet carrier so that the planetary gear mechanism rotates as a unit for direct drive. One significant disadvantage of these mechanisms is that the sun gear must continually handle the full input torque. This requires that the sun gear, the planet gears, and the planet bearings be designed for greater capacity to carry such high loads in order to obtain the desired service life. In addition to such costly overdesign, these mechanisms utilize manually actuated shifter forks which require considerable operator effort, and a substantial number of costly components connected to the shifter forks that require excessive space.
U.S. Pat. No. 4,821,591, issued Apr. 18, 1989 to H. F. Adler, illustrates an axially displaceable ring gear that in one position provides a direct drive mode of operation of a planetary gear mechanism by connecting the input sun gear to the ring gear for joint rotation, and in the other provides a speed reduction ratio by connecting the ring gear to a stationary housing. While this is considerably more desirable from a torque transmission standpoint, the mechanism of U.S. Pat. No. 4,821,591 is more complex and costly in its construction than is desired, and incorporates a conventional unillustrated engaging fork for axially displacing the ring gear.
In view of the above, it would be advantageous to provide a reliable two-speed planetary mechanism that would avoid either the use of a costly and space-consuming shifter fork assembly or the use of a costly friction clutch assembly. Moreover, in order to extend the service lives of the sun and planet gears, it would be beneficial to construct and arrange the elements of the mechanism to transmit torque in dual paths when used the majority of the time in a first speed mode, and to transmit torque in a single path in a less frequently used second speed mode.
It would furthermore be desirable to provide a compact hydraulically-operated device for shifting the ring gear of the planetary gear mechanism positively in at least one axial direction between its two operating modes.
The present invention is directed to overcoming one or more of the problems as set forth above.